I think it is accessed through xdg-protal, and the usage tracking and permission etc. likely happen at that layer.
The xdg-desktop-portal project is adding an age verification portal
(flatpak/xdg-desktop-portal#1922) that needs a data source for the user's age.
userdb already stores personal metadata (emailAddress, realName, location) so birthDate is a natural fit.
I think it is accessed through xdg-protal, and the usage tracking and permission etc. likely happen at that layer.